Little overlap exists in the required capabilities of ²²²Rn (radon) monitors for public health and atmospheric research. The former requires robust, compact, easily transportable instruments to characterise daily to yearly variability >100 Bq m⁻³, whereas the latter requires static instruments capable of characterising sub-hourly variability between 0.1 and 100 Bq m⁻³. Consequently, detector development has evolved independently for the two research communities, and while many radon measurements are being made world-wide, the full potential of this measurement network can't be realised because not all results are comparable. Development of a monitor that satisfies the primary needs of both measurement communities, including a calibration traceable to the International System of Units (SI), would constitute an important step toward (i) increasing the availability of radon measurements to both research communities, and (ii) providing a means to harmonize and compare radon measurements across the existing eclectic global network of radon detectors. To this end, we describe a prototype detector built by the Australian Nuclear Science and Technology Organisation (ANSTO), in collaboration with the EMPIR 19ENV01 traceRadon Project and Physikalisch-Technische Bundesanstalt (PTB). This two-filter dual-flow-loop radon monitor can be transported in a standard vehicle, fits in a 19^′′ instrument rack, has a 30 min temporal resolution, and a detection limit of ∼0.14 Bq m⁻³. It is capable of continuous, long-term, low-maintenance, low-power, indoor or outdoor monitoring with a high sensitivity and an uncertainty of ∼15 % at 1 Bq m⁻³. Furthermore, we demonstrate the successful transfer of an SI traceable calibration from this portable monitor to a 1500 L two-filter radon monitor under field conditions.

In this work, a novel approach for the standardization of low-level ²²²Rn emanation is presented. The technique is based on the integration of a ²²²Rn source, directly, with an α-particle detector, which allows the residual ²²²Rn to be continuously monitored. Preparation of the device entails thermal physical vapor deposition of ²²⁶RaCl₂ directly onto the surface of a commercially available ion implanted Si-diode detector, resulting in a thin-layer geometry. This enables continuous collection of well resolved α-particle spectra of the nuclei, decaying within the deposited layer, with a detection efficiency of approximately 0.5 in a quasi 2π geometry. The continuously sampled α-particle spectra are used to derive the emanation by statistical inversion. It is possible to achieve this with high temporal resolution due to the small background and the high counting efficiency of the presented technique. The emanation derived in this way exhibits a dependence on the relative humidity of up to 15% in the range from 20% rH to 90% rH. Traceability to the SI is provided by employing defined solid-angle α-particle spectrometry to characterize the counting efficiency of the modified detectors. The presented technique is demonstrated to apply to a range covering the release of at least 1 to 210 ²²²Rn atoms per second, and it results in SI-traceable emanation values with a combined standard uncertainty not exceeding 2%. This provides a pathway for the realization of reference atmospheres covering typical environmental ²²²Rn levels and thus drastically improves the realization and the dissemination of the derived unit of the activity concentration concerning ²²²Rn in air.

Radon (²²²Rn) gas is the largest source of public exposure to naturally occurring radioactivity and the identification of radon priority areas is required by the Council Directive 2013/59/Euratom. Radon is also used as a tracer to improve atmospheric transport models and to indirectly estimate greenhouse gas (GHG) fluxes using the Radon Tracer Method (RTM). This method is based on the correlation between atmospheric concentrations of radon and GHG, together with information on the radon flux data. For radiological data, all European countries have installed networks of automatic gamma dose rate monitoring stations and report the real-time information gathered to the European Radiological Data Exchange Platform (EURDEP). So far, atmospheric radon activity concentrations and radon fluxes are not yet reported in EURDEP, nor routinely measured within the European radiological networks although these observations could help to avoid false positives results. Due to above applications, there is a need of building a metrological chain to ensure high quality radon activity concentrations and radon flux measurements. Both climate and radiation protection research communities underline the need for improved traceability in low-level atmospheric radon measurements (Khanbabaee et al., 2021). The EMPIR project 19ENV01 traceRadon ¹ is aimed towards providing the necessary measurement infrastructure and transfer standards to fulfil this need. Results of this project are particularly important for improving independent GHG emission estimates that support national reporting under the Paris Agreement on climate change and for the Council Directive 2013/59/Euratom, thus benefitting two large scientific communities. In this paper, early results, such as new activity standard developments and an overview of commercial and research radon monitors are presented and discussed. These results will feed into the traceRadon project with respect to radionuclide metrology in air and its potential for the improvement of the RTM.